1. Field of the Invention
The present invention relates to a nuclear magnetic resonance imaging apparatus and more particularly, to a nuclear magnetic resonance imaging apparatus suitable for selectively imaging a moving part of a moving object such as a blood.
2. Description of the Prior Art
As a method of measuring a velocity of a moving object such as a human blood by utilizing a nuclear magnetic resonance imaging technology, there is known a phase encode method which images a velocity of a moving object by applying a gradient magnetic field to the moving object, applying after a certain time lapse a gradient magnetic field opposite in polarity to the object, and detecting a phase change of a nuclear spin vector of the object caused by application of the gradient magnetic fields. A gradient magnetic field is a magnetic field slantwise changing its magnetic field intensity along a certain direction. The gradient magnetic field is assumed herein as a magnetic field having a gradient magnetic field strength along the direction of motion of an object. The phase of a nuclear spin vector at each point of the object varies with the intensity and period of an applied magnetic field. In case of a stationary object, since the intensities of first gradient magnetic field and succeeding gradient magnetic field opposite in polarity respectively applied to each point are the same in magnitude and opposite in polarity, the phase change of the nuclear spin vector at the point of the object to be caused by application of both the normal and opposite gradient magnetic fields becomes zero if the respective application periods are the same. In case of a moving object, however, the phase change of a nuclear spin vector at each point of the object varies in accordance with the velocity of the moving object because the intensity of the gradient magnetic field first applied to the point of the object differs in magnitude from that of the gradient magnetic field opposite in phase and applied after a certain time lapse. Thus, it is possible to image the velocity of the moving object by detecting the phase change.
If an object has a moving part and a stationary part, e.g., in the case of a blood vessel wherein a moving blood and a still blood are mixed, and if the latter occupies the most part of the object, it is necessary to extract a signal indicating a phase change in only the moving part to image the velocity of the moving part.
As a first method for this, there is known a method wherein two signals each having phase information both on the stationary and moving parts, are used to obtain a difference between the two different signals, i.e., a phase information on the moving part. According to this method, the signal components of the stationary part contained in each of the two signals are equal to each other, so that a signal representing only the moving part components can be obtained. However, if the stationary part occupies the most of the object as compared with the moving part, the signal components of the moving part is small so that the range of signal change, i.e., a dynamic range, becomes small and that the signal components of the moving part will have a large error even if a slight difference exists between the signal components of the stationary part of the two signals.
As an alternative, there is known a second method wherein a magnetic field of a radio frequency (RF) pulse is applied to an object to rotate the nuclear spin vector of the object by a certain angle relative to Z-axis, i.e., the direction of magnetic field gradient, and an RF magnetic field pulse is applied after a certain time lapse to return the nuclear spin vector of the object to the original angle, with gradient magnetic fields opposite in polarity being applied during application of the RF pulses. According to this method, the nuclear spin vector of an object having no phase change during the period from the time when the first RF pulse was applied until the time when the next pulse is applied is returned to the original angle, whereas the nuclear spin vector having a phase change does not return to the original angle. Thus, a signal representing only the moving part of the object can be detected. The precondition of this method is that the phase of the nuclear spin vector of the stationary part does not change during the period from the time when the first RF pulse was applied until the time when the next RF pulse is applied. However, in practice, the phase of the nuclear spin vector of the stationary part may change due to such as nonuniformity of a static magnetic field, resulting in a problem of an error of a signal representing only the moving part.